Cellular polymers or foams, are multiphase materials that consist of a polymeric matrix and a fluid phase that is typically a gas. Most polymers can be expanded into a foam, but few have been commercially exploited. The two most widely used foam materials in terms of volume are polyurethanes and polystyrenes. Structurally, the cellular polymer system can be described as having either an open-cell or closed-cell geometry. In closed cell foams, the fluid is dispersed in the form of discrete gas bubbles and the polymeric material forms a continuous phase. In open-cell foams, the voids coalesce so that the solid and fluid phases form a continuous network structure.
Foams represent an important class of engineering materials. Because of their low density, excellent mechanical and thermal properties, these materials have been used in a wide variety of applications. Although the majority of modern applications for cellular polymers utilize solid foams, the final product is obtained by processing a liquid foam, e.g., mold filling process, where an understanding of the rheology of the foam is critical.
Rheology is the study of the flow of materials, particularly the plastic flow of solids and the flow of non-Newtonian liquids. If the rheological properties, such as equilibrium modules and zero shear viscosity, of a foaming polymer during the foaming reaction can be determined, an assessment of the evolving flow characteristics, for instance, the mold filling capability, of the polymer can be ascertained. This provides information as to whether or not a particular polymer could be used in a particular process or application, or information on how to improve pre-existing processes already using the polymer.
Despite the widespread use of cellular polymers, only a very limited amount of information exists on the rheological properties of foams, and that is to say, the equilibrium modules and the zero shear viscosity. The majority of the work to date consisted of continuum mechanics-based models for a cellular polymer after the final structure has been formed, or on the load-compression properties of the final foam. Virtually no known work has been done on the aforesaid rheological properties of the liquid cellular polymer during the transient or foaming process. This is largely due to the absence of a reliable rheological instrument capable of handling the unique problems associated with a foaming polymer.
Current commercial rheometer designs are not easily adapted for use with foaming polymers for several reasons. First, the foaming process is a transient process, involving a large volume expansion that is typically ten to twenty times the volume of the original components. This material expansion problem alone eliminates the majority of traditional rheometer designs, which are designed to measure only single phase systems, or are closed system in-line devices.
Secondly, the foaming reaction is quite rapid. Typically, the entire reaction takes two to three minutes. Also, the properties of the foam are changing constantly throughout the foaming reaction. This means that the rheological data must be acquired rapidly. Another problem that must be addressed is the drastic variation in solution viscosity that occurs during the course of the foaming reaction. The viscosity of the foam can change from several poise to several thousand poise in a few minutes.
In an article entitled "Determination of Time Independent Component of the Complex Modulus During Cure of Thermosetting Systems," published in the July 1983 edition of Polymer Engineering and Science, and expressly incorporated by reference herein in its entirety, authors Richard J. Farris and Charles Lee describe an impulse approach to linear visco-elasticity with respect to Narmco 5208 epoxy resin. The impulse approach can be used to separate elastic from viscoelastic contribution to the equilibrium modulus, and to determine whether a material under investigation is a viscoelastic liquid or an elastic solid from the absence or the presence of the equilibrium modulus. This impulse approach, using parallel plates in torsion, has been used to follow the change in the modulus of the Narmco 5208 epoxy resin having a structure that is evolving with time. However, this resin does not generate foam during thermosetting, and there is no suggestion in the article as to how the impulse approach could be used to measure the rheological properties of expanding foam polymers during the foaming reaction.
It is thus one objective of this invention to provide apparatus and methods for measuring and determing certain rheological properties of a foaming cellular polymer during the foaming process.